Ferrocenyl Dendrimers with Ionic Tethers and Dendrons - American

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Ferrocenyl Dendrimers with Ionic Tethers and Dendrons Amalia Rapakousiou, Yanlan Wang, Frida Nzulu, Rodrigue Djeda, Noel̈ Pinaud, Jaime Ruiz, and Didier Astruc* ISM, UMR CNRS No. 5255, University of Bordeaux, 33405 Talence Cedex, France S Supporting Information *

ABSTRACT: Whereas covalently constructed dendrimers are very numerous, there are only a few examples of dendrimers based on ionic bonds with tethers or dendrons. Here such constructions were designed in order to examine their physicochemical consequences: in particular, the electrochemical and redox properties. Two series of secondary polyamine dendrimers containing ferrocenyl and octamethylferrocenyl were synthesized by condensation of polyamine dendrimers, obtained by reduction of the polyazide dendrimers by LiAlH4 or NaBH4 with ferrocenyl aldehyde or octamethylferrocenyl aldehyde, followed by reduction of the resulting polyimine dendrimers. Primary and secondary ferrocenyl polyammonium dendrimer salts have been synthesized by quaternization of the dendritic amines including ferrocenyl secondary polyamines by reaction with a triferrocenyl dendron containing a carboxylic acid as the focal point. Cyclic voltammetry data show the electrostatic effect of the intradendritic secondary ammonium groups and the distinction between intradendritic and peripheral ferrocenyl groups.

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INTRODUCTION Dendrimers now constitute a well-developed field1 that has applications in biomedicine,2 materials science,3 and catalysis.4 Most dendrimers are constructed covalently, but ionic bonding has recently attracted attention for enhanced drug encapsulation and specific physicochemical properties, including mesogens and energy-related aspects.5,6 Here we report arene-centered polyamine dendrimers with ferrocenyl termini and a triferrocenyl dendron with a carboxylic group at the focal point and their ionic assemblies, i.e. dendrimers containing ammonium carboxylate linkers, some of which contain two kinds of functional ferrocenyl termini. The goal was to investigate the possibility of ionic linkages as a viable method for the design of redox sensors and energy-related materials. Among redox-active dendrimers, ferrocenyl derivatives7 are the most practical ones, given the richness and applications of ferrocene chemistry8 that is well illustrated inter alia in this issue and the robustness of the ferrocenyl derivatives.

the dendritic zeroth-generation nonaamine core 3 that is protonated using aqueous HPF6 to give the nonaammonium salt 4 (Scheme 1). The following generation is obtained upon Williamson reaction of the nonachloro core 1 with the phenol triallyl dendron 9 according to a known procedure followed by substitution of the terminal chloride by the azido group11 that is subsequently reduced to the primary amine.12 This gives the first-generation dendrimer 5 containing 27 NH2 termini, and the 27-ammonium species 8 is obtained upon protonation of 5 with aqueous HPF6 or aqueous HCl12 (Scheme 2). These 9NH2- and 27-NH2-terminated dendrimers are used to introduce the ferrocenyl groups upon reaction with either ferrocenyl aldehyde or octamethylferrocenyl aldehyde, although 5 slowly polymerizes in the condensed phase and should be used as freshly prepared. Both ferrocenyl13 and octamethylferrocenyl14 derivatives have already been condensed with polyimine DAB dendrimers by the groups of Jutzi and Casado, respectively, and have also been successfully used here to synthesize new ferrocenyl-terminated dendrimers. The iminoferrocenyl dendrimers obtained using these reactions are reduced to the secondary aminomethylferrocenyl dendrimers 6 and 7 (Scheme 1). The new ferrocenyl-functionalized secondary polyamine dendrimer 6 (Scheme 1) is soluble in toluene, diethyl ether, dichloromethane, and tetrahydrofuran. This amino metallocene was insoluble in acetonitrile, but the ferrocenyl alcohol derived from the reduction of the excess of

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RESULTS AND DISCUSSION Synthesis of New Ferrocenyl Dendrimers Containing Secondary Amine Groups. We begin with the synthesis of arene-centered dendrimers according to our classic CpFe+induced nonaallylation of mesitylene in [FeCp(η6-1,3,5C6H3(CH3)3)][PF6]9 according to a 1→3 connectivity10 providing the nonaallyl core11 followed by hydrosilylation with chloromethyldimethylsilane, substitution of the terminal chloro group in 1 by reaction with sodium azide to give the nonaazide 2,11 and reduction of the azido groups in 2 to primary amine termini.12 This sequence of reactions provides © XXXX American Chemical Society

Special Issue: Ferrocene - Beauty and Function Received: July 2, 2013

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dx.doi.org/10.1021/om400650s | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1

ferrocenyl dendrimer is more aerobically stable than those containing octamethylferrocenyl termini in solution, because the eight methyl ferrocene substituents facilitate oxidation to ferrocenium derivatives by oxygen. These two secondary polyamine dendrimers 6 and 7 were characterized by 1H NMR and 13C NMR, IR, DOSY NMR, cyclic voltammetry, and elemental analyses. Synthesis of the New Triferrocenyl Dendrons. A triferrocenyl dendron containing a carboxylic acid group at the focal point, 13, used for the quaternization reaction, was synthesized according to Scheme 3. The synthesis of this dendron starts by the known CpFe+-induced triallylation and exocyclic C−O cleavage in p-methylethoxytoluene obtained by

ferrocenyl aldehyde in the reaction solution is soluble in acetonitrile. We acted on this solubility difference to purify the products by precipitation of the crude reaction from acetonitrile with a small amount of dichloromethane. For the synthesis of the octamethylferrocenyl-functionalized dendrimer 7, instead of ferrocenyl aldehyde, octamethylferrocenyl aldehyde15 was used with the same route as the synthesis of the dendrimer ferrocenyl moieties, except for the process of reduction from imine to amine, which was achieved in this case by using NaBH4 in a MeOH/THF mixture as solvent. The functionalized secondary polyamine dendrimer 7 with attached octamethylferrocenyl termini is soluble in toluene, diethyl ether, dichloromethane, and tetrahydrofuran. The polyamine B


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Scheme 2

reaction of ethanol and sodium carbonate with [FeCp(η6-pCH3C6H4Cl)][PF6]. The phenol triallyl dendron 9 obtained in this way was hydrosilylated with dimethylsilylferrocene to give the known complex 10. 9c Then an S N 2 reaction of BrCH2COOMe with the phenol derivative 10 in the presence of K2CO3 provided the new ester 11. Saponification15 of the ester group was conducted with NaOH in H2O/dioxane. The sodium carboxylate dendron product was obtained as a red oil in 90% yield, and acidification was carried out upon addition of aqueous HCl to give the ether-soluble carboxylic acid dendron 13 (Scheme 3). These new triferrocenyl dendrons were

characterized by 1 H NMR and 13 C NMR, IR, mass spectrometry, cyclic voltammetry, and elemental analyses. Synthesis of the Primary Polyammonium Dendrimers. The nonaammonium hexafluorophosphate dendrimer 4 was synthesized from the primary polyamine nona dendrimer 3 in dichloromethane by dropwise addition of aqueous HPF6. The turbid solution was stirred for an additional 30 min. The same procedure was used for the synthesis of the ionic dendrimer 8 from the polyamine precursor 5 with aqueous HCl. These dendrimers were soluble only in very polar solvents such as water, DMF, and DMSO. In 1H NMR and 13C C


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Scheme 3

Chart 1

D


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Chart 2

were soluble in dichloromethane, toluene, and THF. The dendron 13 with carboxylic acid at the focal point was ether soluble, as well as the secondary polyamine ferrocenyl and octamethylferrocenyl dendrimers 6 and 7. On the other hand, the sodium carboxylate dendron 12 was insoluble in ether. The same phenomenon is noticed in the polyammonium carboxylate dendrimers that are not ether soluble. Finally, as the ammonium carboxylate functions are located at the interior of the dendrimers, the ionic groups are shielded, and they cause these ionic dendrimers to be insoluble in polar solvents such as water, acetone, and acetonitrile. Products 14−17 were characterized by 1H NMR and 13C NMR, IR, DOSY NMR, cyclic voltammetry, and elemental analyses. NMR Spectroscopy. NMR spectroscopy proved to be a key tool for this study, confirming the proposed structures.

NMR in D2O, all of the characteristic peaks of the dendrimers are observed. Products 4 and 8 were also analyzed by IR and elemental analysis. Synthesis of the Supramolecular Ionic Ferrocenyl Dendrimers. The construction of higher generations of ferrocenyl dendrimers, generations 1 (products 14, 16, and 17) and 2 (product 15), were achieved in a facile and clean acid−base reaction (Charts 1−3). One equivalent of the primary and secondary polyamine dendrimer precursors, generations 0 and 1, were mixed with 9 and 27 equiv of the dendron 13 in dichloromethane at room temperature, yielding orange sticky oils. The solubility of the final supramolecular products is influenced as expected by the periphery of the dendrimers that contains 27 (products 14, 16, and 17) and 81 (product 15) ferrocenyl moieties, and these final dendrimers E


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Chart 3

Table 1. 1H NMR, 13C NMR, and IR Data of Dendrons 11−13 and Dendrimers 3−8 and 14−17 CH2COOR 1H/13C (ppm)

compd a

dend-COOMe 11 dend-COONa 12a dend-COOH 13a G0-NH2 3a G1-NH2 5a G0-NH3PF6 4b G1-NH3Cl 8b G1-NH3-Fc27 14a G2-NH3-Fc81 15a G0-NH-Fc 6a G0-NH-Fc# 7a G1-NH2-Fc36 16a G1-NH2-Fc#9Fc27 17a a

SiCH2NHx 1H/13C (ppm)

HxNCH2Cp 1H/13C (ppm)

4.66/65.10 4.29/64.00 4.56/66.29

4.38/67.98 4.41/66.51

4.42/65.99 4.42/67.85

2.04/29.90 2.06/30.00 2.34/27.76 2.43/27.86 2.27/25.58 2.30/27.96 2.10/29.70 2.04/29.72 2.20/30.00 2.22/29.70

3.54/53.15 3.48/50.20 3.89/50.38 3.97/51.19

IR KBr (cm−1) 1765; 1633 1722; 3365; 3350; 3431; 3425 3445; 3452; 3448; 3456; 3452; 3461;

1607 1607 1570 1581 830 1636 1643 1592 1593 1641 1630

NMR in CDCl3 at 25 °C. bNMR in D2O at 25 °C.

CH2COOMe group and the methyl group at 3.86 ppm appeared, integrating for two and three protons, respectively. After the saponification reaction, the dendron 12 showed the CH2COONa signal at lower field as expected around 4.29 ppm, whereas the signal of the methyl group disappeared. The acidification reaction that turned the focal point into a

Table 1 shows the comparative NMR (and IR) data for the dendronic carboxylic acid 13, the corresponding carboxylate sodium salt 12, the dendritic primary and secondary polyamines (3 and 5−7), and the dendritic primary and secondary polyammonium carboxylates (4, 8, and 14−17). For the dendron 11, the 1H NMR signals at 4.66 ppm of the F


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For the primary polyamines 3 and 5 two broad bands were observed in the region 3400−3300 cm−1 (−NH2 stretching), whereas for the secondary polyamines 6 and 7 one broad band was observed in this region (−NH− stretching). For the primary and secondary polyammonium dendrimers (products 5, 8, and 14−17), one broad absorption was observed in the region 3420−3460 cm−1 due to −NH3+ or −NH2+− stretching (see Table 1). DOSY NMR. DOSY (diffusion-ordered spectroscopy) experiments were conducted for the dendrimers 6, 7, 14, 16, and 17. The main goal of these experiments was to measure the diffusion coefficient D and determine the size of the dendrimer in solution. The latter also reflects the purity of the products. The D value allows calculation of the hydrodynamic diameter of a molecule. Dendrimers are regarded as spherical molecular objects and characterized by an apparent diffusion coefficient. The Stokes−Einstein law, D = kBT/6πηrH, gives an estimate for the diameter of the molecule, where D is the diffusion coefficient, kB the Boltzmann constant, T the absolute temperature, η the solvent viscosity, and rH the hydrodynamic radius of the species (Table 2). The diameter values that are

carboxylic acid group was checked by the downfield shift of the CH2COOH signal at 4.56 ppm. 13C NMR spectroscopy also confirms the above structures (see the Supporting Information). The syntheses of the dendrimers 6 and 7 were also monitored by 1H NMR. The resulting imine dendrimers resulting from the condensation reactions between 3 and the ferrocenyl and octamethylferrocenyl aldehydes showed the appearance of the −NCH− signal at 8.1−8.2 ppm. After reduction to the corresponding secondary amines, this signal completely disappeared, and the −NHCH 2 −Cp signal appeared at 3.54 and 3.48 ppm, respectively. The corresponding signals of the ferrocene and octamethylferrocene moieties also confirmed the structures of the dendrimers 6 and 7. The ionic dendrimers 4 and 8 were dissolved in D2O, but their solubility in CDCl3 (for the sake of comparison) was not determined. Nevertheless, the −CH2NH3+ signals showed an important downfield shift. In the case of the primary polyammonium ferrocenyl dendrimers 14 and 15, by comparison with the primary polyamine precursors 3 and 5 and dendron 13, the −CH2NH3+ proton signal was deshielded by about 0.25 ppm due to the presence of a nearby positive charge, and the signal of the −CH2COO− protons was found to be shielded by about 0.17 ppm due to the presence of a nearby negative charge. These shifts in comparison to the neutral termini are in agreement with the formation of the ionic ammonium carboxylate linker. In the case of the secondary polyammonium dendrimers 16 and 17, in comparison to the precursors 6 and 7 and dendron 13, the differences are slightly smaller, as −CH2NH2+ protons are deshielded by about 0.10 and 0.18 ppm, respectively and the −CH2COO− protons are shielded by 0.14 ppm. This decrease of the charge effect is taken into account by the fact that the secondary ammonium groups are larger than the primary groups, resulting in a dilution of the positive charge and weaker (longer) ionic bond with the carboxylate. On the other hand, the −NH2+CH2−Cp signal of the secondary polyammonium dendrimers 16 and 17 are much more shifted (0.3 and 0.5 ppm) toward lower field than the signals of the other protons. Interestingly, the 13C NMR spectra show the opposite effect for the −CH2NH3+ (shielded compared to −CH2NH2) and −CH2COO− carbons (deshielded in comparison to −CH2COOH) in comparison to proton signals in dendrimers 14 and 15 (see Table 1). Finally, for the ionic dendrimers, the corresponding proton signals of the carboxylates are found at an intermediate region between those of the dendronic acid 13 and those of the dendronic sodium carboxylate salt 12, probably because of the hydrogen bonding between the primary or secondary ammonium group with the carboxylate group. Infrared Spectroscopy. The infrared spectra also provided valuable information on the new series of dendrons and dendrimers. Dendron 11 has the characteristic absorption of the ester group at 1765 cm−1, and in dendron 13 the presence of a focal point consisting of the carboxylic acid was checked by the frequencies related to the carbonyl group at 1722 and 1607 cm−1 (and a broad band due to the O−H stretching at 3400 cm−1). For the dendron 12 the single carbonyl band was shifted to the region between the two, at 1633 cm−1 due to the carboxylate absorption (both resonance forms equally contribute to the ground state of the molecule). Similarly, for the dendritic polyammonium carboxylates 14−17, a strong band due to −COO− stretching was found in the region 1630−1643 cm−1.

Table 2. Calculated Diffusion Coefficients and Hydrodynamic Radii Obtained by DOSY NMR compd

Da (±0.1), 10−10 m2/s

rHb (±0.1), nm

G0-NH-Fc 6 G0-NH-Fc# 7 G1-NH3-Fc27 14 G1-NH2-Fc36 16 G1-NH2-Fc#9Fc27 17

1.947 1.768 1.044 1.261 1.216

2.08 2.29 3.87 3.21 3.33

D is the diffusion coefficient, measured in CDCl3 at 25 °C, brH is the hydrodynamic radius, calculated using the Stokes−Einstein equation.

a

obtained include peripheral solvation of the products by solvent molecules in solution. This method was very useful for a clear comparison between the generations of the dendrimers and provided evidence for the formation of distinct ionic assemblies. For the nonionic dendrimers 6 and 7 (generation 0), the calculated rH values are 2.08 and 2.29 nm, respectively, which shows the effect of the presence of methyl groups in 7 and provides a hydrodynamic radius that is 0.21 nm larger herewith. When the latter products were allowed to react with the dendron 13, giving the next generation (generation 1) dendrimers 16 and 17, it was found that the sizes of the obtained dendritic ionic assemblies represented by rH are 3.21 and 3.33 nm, respectively. This finding that 17 has a larger rH value than 16 is in agreement with the relative sizes of their precursors, respectively 6 and 7. Finally, DOSY NMR, apart from providing valuable data such as the diffusion coefficients of the new dendrimers, also gives clear evidence of the size progression upon an increase of the dendrimer generation. It also allows comparing the sizes of the dendrimers (ionic and nonionic) that contain different metallocenes and distinguishing between the assemblies of primary and secondary polyammonium dendrimers of the same generation. Cyclic Voltammetry.16 The new dendron 13 and the series of polyamine and polyammonium ferrocenyl dendrimers were studied by cyclic voltammetry using decamethylferrocene (FeCp*2) as the internal reference,17 although ferrocene was used in the cases of the octamethylferrocenyl dendrimers 7 and 17. The cyclic voltammograms were recorded in dichloromethane, and the E1/2 data (measured vs FeCp*2) are gathered G


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Table 3. Redox Potentials and Chemical (ia/ic) and Electrochemical (Epa − Epc = ΔE) Reversibilities for Compounds 6, 7, and 13−17α FeIII/II dendron compd

E1/2, V

ΔE, V

ia/ic

13 6 7 14 15 16 17

0.54

0.068

0.76

0.52 0.53 0.54 0.52

0.015 0.012 0.065 0.025

Fe#III/II dendrimer

FeIII/II dendrimer

0.57 0.42 1 1

E1/2, V

ΔE, V

ia/ic

0.55

0.065

1

0.62

0.060

E1/2, V

ΔE, V

ia/ic

0.10

0.040

1

0.27

0.030

0.9

1

α

Conditions: supporting electrolyte, [n-Bu4N][PF6] 0.1 M; solvent, CH2Cl2; reference electrode, Ag; working and counter electrodes, Pt; scan rate, 0.2 V/s; internal reference, FeCp*2.

in Table 3. For all of the dendrons and dendrimers, a single oxidation wave is observed for all the ferrocenyl groups, which appear seemingly equivalent, due to the weakness of the electrostatic factor between the redox sites of the dendron and metallodendrimers, as their redox sites are far away from one another, being separated by several bonds.18 This single wave for all these three products is chemically and electrochemically reversible, although this reversibility is sometimes more or less perturbed by adsorption of these macromolecules onto the electrode. The electrochemical reversibility involving all the redox groups and signifying fast heterogeneous electron transfer is due to two factors that probably simultaneously occur: (i) fast rotation of the metallodendrimer within the electrochemical time scale, whereby all redox groups come in turn close to the electrode,19 and (ii) electron hopping among the redox sites borne by flexible tethers, allowing hopping at an optimal distance and mutual orientation.20 For dendron 13 and dendrimer 6, the FeIII/II oxidation potentials of the ferrocenyl redox centers are about 0.54 V, whereas for dendrimer 7 this potential is 0.1 V. This cathodic shift is accounted for by both the electron-releasing property of the eight methyl substituents of the ferrocenyl groups that facilitate oxidation to ferrocenium. For primary polyammonium dendrimers 14 and 15, the oxidation potentials are about the same as that of the dendron (only 20 mV less positive). The ferrocenyl centers at the periphery of the dendrimers are not influenced by the ammonium carboxylate linkages that are located in the dendritic interior. In the case of secondary polyammonium dendrimers with two kinds of ferrocenyl centers (product 16) and the mixed ferrocenyl−octamethylferrocenyl dendrimer 17, two waves are observed corresponding to each type of ferrocenyl redox center. For products 16 and 17, oxidation of the peripheral ferrocenes also is recorded at 0.54 V, because they are not influenced by the remote interior ammonium carboxylate linkages. Oxidation of the interior ferrocenyl groups that are close to the ammonium groups now takes place at much more positive potentials, which is due to the interaction with and proximity of the ammonium cation. This trend is more marked for 17, because the ammonium cation stabilizes the electron-rich octamethylferrocenyl group, which is oxidized at a potential 170 mV more positive (Figure 1). The adsorption phenomenon during cyclic voltammetry of cationic dendrimers is much more marked for ionic dendritic assemblies than for neutral species. Indeed, for the dendron 13 and the generation-0 dendrimers 6 and 7, adsorption is not observed, but in the case of the ionic compounds 14−17 adsorption is strong. Especially for compounds 14 and 15 adsorption onto the electrode is observed by scanning around

Figure 1. CV of 17 in CH2Cl2 solution. Conditions: reference electrode, Ag; working and counter electrodes, Pt; scan rate, 0.2 V/s; supporting electrolyte, [n-Bu4N][PF6].

the oxidation potentials of the peripheral ferrocenyl groups, which confirms the formation of ionic generation-1 and -2 dendrimers and the existence of electrostatic forces. For the ionic compounds 16 and 17 adsorption is also highly marked. On the other hand, the dendron 13 and dendrimers 6 and 7 do not adsorb and cannot modify the Pt electrodes. This is not the case for dendrimers 14 and 16, which deposit onto the electrode surfaces upon scanning toward potentials that are positive to ferrocenyl oxidation. Modification of electrodes with films of dendrimers containing reversible redox systems has been successful, resulting in detectable electroactive material. This electrochemical behavior of modified electrodes was studied in dichloromethane containing only the supporting electrolyte (Figures 2 and 3). A well-defined, symmetrical redox wave is observed, which is characteristic of a surface-confined redox couple, with the expected linear relationship of peak current with potential sweep rate v.21 Repeated scanning does not change the voltammograms, demonstrating that the modified electrodes are stable to electrochemical recycling. However, splitting between oxidation and reduction peaks is observed (ΔE = 40 mV), which suggests that a structural reorganization takes place within the electrochemical redox process within these ionic dendritic assemblies. Values of the full width at half-maximum for compounds 14 and 16 were measured at a scan rate of 100 mV/s and found to be ΔEfwhm = 150 and 140 mV, respectively, suggesting the existence of H


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peripheral ferrocenyl layers such as those synthesized here. The effect of the intradendritic positive charges of the ammonium groups clearly appears on the NMR chemical shifts and electrochemical potentials (E1/2 values) of the inner ferrocenylmethyl groups. Interestingly, this electrostatic effect is more pronounced on the octamethylferrocenyl layer than on the parent ferrocenyl layer, probably because of the electronrich methyl substituents in comparison to parent ferrocenyl sites. The E1/2 value of the inner ferrocenyl layer is sufficiently shifted anodically to show a CV wave (shoulder) that is slightly distinct from that of the outer ferrocenyl layer. When the ferrocenyl substituents are different (H vs CH3), the two separate CV waves clearly appear with relative intensities corresponding to their ratio (3/1). Finally, a characteristic of all the ionic ferrocenyl dendrimers is their strong ability to adsorb on electrodes and form derivatized electrodes that is considerably more marked than that of their neutral constituents. Potential applications are, for instance, the dendritic encapsulation of ionic or polar biomedical molecules in hydrophobic media and recyclable redox sensors.

Figure 2. Cyclic voltammograms of 14 in CH2Cl2 solution containing 0.1 M [n-Bu4N][PF6]: (a) in solution; (b) intensity as a function of scan rate (linearity shows the expected behavior of an absorbed dendrimer); (c) modified Pt electrode at various scan rates in a CH2Cl2 solution (containing only the supporting electrolyte).

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EXPERIMENTAL SECTION

General Data. For general data including solvents, apparatuses, compounds, reactions, spectroscopy, and CV, see the Supporting Information . Gn indicates the generation number n. G0-9-dend-NHCH2-Fc (6). To a solution of ferrocenyl aldehyde (0.077 g, 0.358 mmol, 1.02 equiv/branch) in 30 mL of dry toluene was added dropwise at 70 °C a solution of polyamine dendrimer 3 (0.050 g, 0.033 mmol) in 20 mL of dry toluene under nitrogen. The reaction mixture was heated at 75 °C for 12 h. The red solution was then evaporated to dryness under reduced pressure, the resulting reddish solid containing Gn-3n+2-dend-NCH-Fc was dissolved in 20 mL of THF, and a suspension of LiAlH4 (2 equiv/branch) in 20 mL of THF was added slowly at 0 °C. After the addition, the mixture was stirred for 16 h at 50 °C under nitrogen. To the reaction solution at 0 °C was added dropwise H2O (6 equiv/branch), and the mixture was stirred for 20 min. After drying under vacuum, the crude product was washed with acetonitrile in order to remove the excess of ferrocene aldehyde/ methanol. Further purification was achieved from precipitation in acetonitrile, and the product was obtained as an orange sticky oil. Yield: 70%. 1 H NMR (CDCl3, 300 MHz): δ (ppm) 7.01 (3H, CH core), 4.22 (18H, Cp sub), 4.15 (45H, Cp free), 4.15 (18H, Cp sub), 3.54 (18H, CpCH2NH), 2.10 (18H, SiCH2NH), 1.63 (18H, CqCH2CH2CH2Si), 1.11 (18H, CqCH2CH2CH2Si) 0.54 (18H, CqCH2CH2CH2Si), 0.03 (s, 54H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 145.7 (Cq of arom core), 128.7 (CH of arom core), 86.3, 68.6, 68.4, 67.7 (Cp-C), 53.2 (CH2-Cp), 43.1 (CqCH2CH2CH2Si), 42.1 (CqH2CH2CH2Si), 29.7 (Si(CH 3 ) 2 CH 2 NH), 17.7 (C q CH 2 CH 2 CH 2 Si), 15.1 (C q CH 2 CH 2 CH 2 Si), −3.8 (Si(CH 3 ) 2 ). Anal. Calcd for C162H237Si9N9Fe9: C, 63.46; H, 7.79.. Found: C, 63.33; H, 7.80. G0-9-dend-NHCH2-Fc# (7). To a solution of octamethylferrocene (54.8 mg, 0.16 mmol, 1.2 equiv/branch) in 30 mL of dry toluene was added dropwise at 70 °C a solution of nonaamine dendrimer (19.2 mg, 0.015 mmol) in 20 mL of dry toluene under nitrogen. The reaction mixture was heated to 75 °C for 16 h. The red solution was then evaporated to dryness under reduced pressure, and the resulting reddish solid containing G0-dend-NCH-Fc# was washed with 20 mL of CH3CN three times to remove the excess octamethylferrocene aldehyde/methanol. The resulting reddish solid was dried under reduced pressure and dissolved in a mixture of THF (10 mL) and CH3OH (10 mL), and then NaBH4 (3 equiv/branch) was slowly added at 0 °C. After the addition, the mixture was stirred for 30 min at 0 °C under nitrogen. The solvent was removed under reduced pressure. To the remaining solid was added dropwise H2O (30 mL), and the compound was extracted three times with distilled diethyl ether. The combined organic phase was dried under anhydrous

Figure 3. Cyclic voltammograms of 16 in CH2Cl2 solution containing 0.1 M [n-Bu4N][PF6]: (a) in solution; (b) intensity as a function of scan rate (linearity shows the expected behavior of an absorbed dendrimer); (c) modified Pt electrode at various scan rates in CH2Cl2 (solution containing only the supporting electrolyte).

repulsive forces between the ferrocenyl sites attached onto the electrode surface.22 These Pt electrodes that are modified with dendrimers 14 and 16 are durable and reproducible, as no loss of electroactivity is observed after scanning several times or after standing in air for several days. The surface coverages of the electroactive ferrocenyl sites of the modified electrodes of metallodendrimers 14 and 16 are Γ = 1 × 10−11 and 1.7 × 10−11 mol cm−2, respectively. Concluding Remarks. The assembly of ionic dendrons and dendrimers represents a convenient way to synthesize large dendrimers with defect-free periphery insofar as the dendrimer purity solely depends on the purities of the dendrons and dendritic cores. The ionic linkers at the dendritic interiors are shielded so that the properties of these dendrimers essentially depend on the periphery. Of specific interest are ionic ferrocenyl dendrimers containing both intradendritic and I


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17.87 (CqCH2CH2CH2Si), 17.39 (CqCH2CH2CH2Si), −2.08 (Si(CH3)). MS (m/z): calcd for C55H72O3Si3Fe3 1032.2832, found 1032.7785. Dendron 12. 1H NMR (CDCl3, 300 MHz): δ (ppm) 7.15, 6.81 (4H, arom), 4.34 (6H, Cp sub), 4.29 (2H, CH2COONa), 4.12 (15H, Cp free), 4.08 (6H, Cp sub), 3.86 (3H, CH2COO−CH3), 1.62 (6H, C q CH 2 CH 2 CH 2 Si), 1.12 (6H, C q CH2 CH 2 CH2 Si), 0.64 (6H, CqCH2CH2CH2Si), 0.19 (18H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 167.16 (COONa), 154.07 (OCq arom), 140.17 (Cq arom), 127.61, 113.71 (CH arom), 73.00, 70.72 (CH of Cp sub), 70.98 (Cq of Cp sub), 68.17 (Cp free), 64.00 (CH2COONa), 42.89 (CqCH2CH2CH2Si), 41.82 (CqCH2CH2CH2Si), 17.76 (CqCH2CH2CH2Si), 17.23 (CqCH2CH2CH2Si), −1.79 (Si(CH3)). Dendron 13. 1H NMR (CDCl3, 300 MHz): δ (ppm) 7.22, 6.90 (4H, arom), 4.56 (2H, CH2COOH), 4.35 (6H, Cp sub), 4.13 (15H, Cp free), 4.06 (6H, Cp sub), 1.65 (6H, CqCH2CH2CH2Si), 1.17 (6H, CqCH2CH2CH2Si), 0.65 (6H, CqCH2CH2CH2Si), 0.21 (18H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 174.41 (COOH), 155.11 (OCq arom), 141.32 (Cq arom), 127.63, 114.42 (CH arom), 73.19, 70.90 (CH of Cp sub), 71.51 (Cq of Cp sub), 68.43 (Cp free), 6 6. 29 ( C H 2 C O O H ) , 4 3 . 3 3 ( C q CH 2 C H 2 C H 2 S i ) , 4 2 . 0 4 (CqCH2CH2CH2Si), 18.26 (CqCH2CH2CH2Si), 17.74 (CqCH2CH2CH2Si), −1.87 (Si(CH3)2). MS (m/z): calcd for C54H70O3Si3Fe3 1018.2681, found 1018.2715. Anal. Calcd for C54H70O3Si3Fe3: C, 63.65; H, 6.92. Found: C, 63.32; H, 7.07. General Synthesis of Ionic Polyammonium Carboxylate Dendrimers. To a stirred solution of secondary or primary polyamine dendrimer in 10 mL of distilled dichloromethane was added dropwise a solution of triferrocenyl carboxylic acid (1 equiv/branch) in 10 mL of dichloromethane, and the mixture was stirred for 20 min. The solvent was removed under vacuum, and the compound was obtained as an orange-yellow sticky oil in quantitative yield. Dendrimer 14. The ionic dendrimer 14 was synthesized from 3 (10 mg, 0.008 mmol, 1 equiv) and 13 (71.5 mg, 0.070 mmol, 9 equiv) using the general synthesis of ionic polyammonium carboxylate dendrimers. 1 H NMR (CDCl3, 300 MHz): δ (ppm) 7.17 (18H, arom), 7.02 (3H, arom core), 6.86 (18H, arom), 4.38 (18H, CH2COO−), 4.33 (54H, Cp sub), 4.12 (135H, Cp free), 4.05 (54H, Cp sub), 2.27 (18H, SiCH 2 NH 3 + ), 1.63 (72H, C q CH 2 CH 2 CH 2 Si), 1.15 (72H, CqCH2CH2CH2Si), 0.64 (72H, CqCH2CH2CH2Si), 0.20, 0.10 (216H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 174.88 (COO−), 155.62 (OCq arom of dendron), 140.46 (Cq arom), 127.48, 114.35 (CH arom), 72.91, 71.29, 70.75, 68.25 (Cp−C), 67.98 (CH2COO−), 43.24 (CqCH2CH2CH2Si), 42.19 (CqCH2CH2CH2Si), 25.58 (Si(CH 3 )2 CH2 NH3 + ), 18.14 (C q CH2 CH 2 CH2 Si), 17.66 (CqCH2CH2CH2Si), −1.85, −3.99 (Si(CH3)2). Anal. Calcd for C540H777Si36Fe27N9O27: C, 62.69; H, 7.58. Found: C, 63.02; H, 7.73. Dendrimer 15. The ionic dendrimer 15 was synthesized from 5 (10 mg, 0.002 mmol, 1 equiv) and 13 (49.5 mg, 0.049 mmol, 27 equiv) using the general synthesis of ionic polyammonium carboxylate dendrimers. 1 H NMR (CDCl3, 300 MHz): δ (ppm) 7.17 (72H, arom), 6.95 (3H, arom core), 6.85 (72H, arom), 4.41 (54H, CH2COO−), 4.33 (162H, Cp sub), 4.12 (405H, Cp free), 4.05 (162H, Cp sub), 3.55 (CH2OAr), 2.30 (54H, SiCH2NH3+), 1.64 (234H, CqCH2CH2CH2Si), 1.13 (234H, CqCH2CH2CH2Si), 0.65 (72H, CqCH2CH2CH2Si), 0.21, 0.05 (216H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 174.10 (COO−), 154.87 (OCq arom of dendron), 141.22 (Cq arom), 127.54, 114.34 (CH arom), 72.92, 71.28, 70.62, 68.09 (Cp-C), 66.51 (CH2COO−), 43.27 (CqCH2CH2CH2Si), 42.09 (CqCH2CH2CH2Si), 27.96 (Si(CH 3 )2 CH2 NH3 + ), 18.07 (C q CH2 CH 2 CH2 Si), 17.60 (CqCH2CH2CH2Si), −1.91, −3.40 (Si(CH3)2). Anal. Calcd for C1719H2487Si117Fe81N27O90·2CH2Cl2: C, 62.74; H, 7.63. Found: C, 62.64; H, 7.89. Dendrimer 16. The ionic dendrimer 16 was synthesized from 6 (10 mg, 0.003 mmol, 1 equiv) and 13 (29.9 mg, 0.029 mmol, 9 equiv) using the general synthesis of ionic polyammonium carboxylate dendrimers.

Na2SO4. After evaporation, the compound was obtained as a yellow solid without further purification. Yield: 77%. 1 H NMR (CDCl3, 300 MHz): δ (ppm) 6.92 (3H, CH core), 3.48 (18H, NHCH2C5(CH3)4), 3.28 (9H, C5(CH3)4H), 2.04 (18H, SiCH2NH), 1.83, 1.79, 1.77, 1.71 (216H, C5(CH3)4), 1.58 (18H, CqCH2CH2CH2Si), 1.04 (18H, CqCH2CH2CH2Si), 0.47 (18H, CqCH2CH2CH2Si), 0.05 (54H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 145.7 (Cq of arom core), 128.7 (CH of arom core), 80.5, 80.0, 79.8, 79.7, 79.4, and 70.5 (Cp-C), 50.2 (Cp-CH2NH), 43.8 (C q CH 2 CH 2 CH 2 Si), 42.0 (C q CH 2 CH 2 CH 2 Si), 29.7 (Si(CH3)2CH2NH), 17.8 (CqCH2CH2CH2Si), 15.1 (CqCH2CH2CH2Si), 11.0, 10.0, 9.9, 9.4 (C5(CH3)4), −4.0 (Si(CH3)). Anal. Calcd for C234H381Si9Fe9N9: C, 68.95; H, 9.43. Found: C, 68.71; H, 9.14. G0-NH3+PF6− (4). To a stirred solution of 3 (100 mg, 0.078 mmol) in 20 mL of distilled dichloromethane was added dropwise aqueous HPF6 (1.1 equiv/branch) diluted in 3 mL of dichloromethane. Then a white precipitate started to form, but the product was stirred for an additional 30 min. The solution was evaporated, and the white solid was washed with acetone. The water-soluble white product 4 was obtained quantitatively. 1 H NMR (D2O, 300 MHz): δ (ppm) 6.92 (3H, CH core), 2.26 (18H, SiCH2NH3+), 1.55 (18H, CqCH2CH2CH2Si), 0.99 (18H, CqCH2CH2CH2Si) 0.50 (18H, CqCH2CH2CH2Si), 0.03 (s, 54H, Si(CH3)2). 13C NMR (D2O, 75 MHz): δ (ppm) 145.72 (Cq arom), 121. 65 (CH a rom), 43. 66 (C q C H 2 CH 2 C H 2 S i ) , 4 1 . 1 1 (C q CH 2 CH 2 CH 2 Si), 27.76 (Si(CH 3 ) 2 CH 2 NH 2 + ), 17.41 (CqCH2CH2CH2Si), 13.60 (CqCH2CH2CH2Si), −4.95 (Si(CH3)). Anal. Calcd for C63H156Si9N9P9F54·2H2O: C, 28.73; H, 6.13. Found: C, 28.82; H, 6.17. G1-NH3+Cl− (8). To a stirred solution of 5 (50 mg, 0.009 mmol) in 20 mL of distilled dichloromethane was added dropwise aqueous HCl (1.2 equiv/branch) diluted in 5 mL of dichloromethane. Then a white precipitate started to form, but the product was stirred for an additional 30 min. The solution was evaporated, and the white solid was washed with acetone. The water-soluble white waxy product 8 was obtained quantitatively. 1 H NMR (D2O, 300 MHz): δ (ppm) 7.15, 6.72 (39H, CH core), 3.73 (18H, CH2OAr), 2.43 (54H, SiCH2NH3+), 1.88 (72H, CqCH2CH2CH2Si), 1.16 (72H, CqCH2CH2CH2Si), 0.63 (72H, CqCH2CH2CH2Si), 0.13 (216H, Si(CH3)2). 13C NMR (D2O, 75 MHz): δ (ppm) 158.72 (OCq arom of dendron), 139.56 (Cq arom), 127.23, 113.77 (CH arom), 43.16 (CqCH2CH2CH2Si), 41.69 (C q CH 2 CH 2 CH 2 Si), 27.86 (Si(CH 3 ) 2 CH 2 NH 3 + ), 17.60 (CqCH2CH2CH2Si), 14.56 (CqCH2CH2CH2Si), −4.19 (Si(CH3)2). Anal. Calcd for C288H624Si36N27O9Cl27·5H2O: C, 51.88; H, 9.59. Found: C, 51.60; H, 9.57. Triferrocenyl Carboxylic Acid (13). To a solution of 10 (0.351 g, 0.365 mmol) in 40 mL of acetonitrile were added K2CO3 (0.101 g, 0.731 mmol) and methyl bromoethanoate (0.084 g, 0.548 mmol), and the resulting solution was heated at 90 °C for 16 h under a nitrogen atmosphere. After the solvent and excess methyl bromethanoate were removed, the crude product was dissolved in 100 mL of dichloromethane and the solution filtered through Celite. The solvent was removed under vacuum, and the product was obtained as an orange oil. Then product 11 (0.360 g, 0.35 mmol) was dissolved in 18 mL of dioxane, and a solution of NaOH (56 mg, 1.39 mmol) in 5 mL of H2O was added dropwise. The solution was stirred for 16h at room temperature. Then, the solution was evaporated, and the product was washed in water and extracted from dichloromethane. Acidification of 12 in dichloromethane/water with aqueous HCl gave product 13 as an orange shiny oil. Yield: 90%. Dendron 11. 1H NMR (CDCl3, 300 MHz): δ (ppm) 7.20, 6.90 (4H, arom), 4.66 (2H, CH2COOMe), 4.35 (6H, Cp sub) 4.13 (15H, Cp free), 4.06 (6H, Cp sub), 3.86 (3H, CH2COO−CH3), 1.65 (6H, C q CH 2 CH2 CH 2 Si), 1.15 (6H, C q CH 2 CH 2 CH 2 Si), 0.65 (6H, CqCH2CH2CH2Si), 0.21 (18H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 168.48 (COOMe), 155.08 (OCq arom), 141.20 (Cq arom), 127.32, 113.91 (CH arom), 72.75, 70.36 (CH of Cp sub), 71.16 (Cq of Cp sub), 67.81 (Cp free), 65.10 (CH2COOMe), 52.17 (COOCH3), 43.24 (CqCH2CH2CH2Si), 41.80 (CqCH2CH2CH2Si), J


Organometallics

Article

H NMR (CDCl3, 300 MHz): δ (ppm) 7.21, 6.89 (39H, arom), 4.41 (18H, CH2COO−), 4.35, 4.14, 4.07 (324H, Cp), 3.89 (18H, + NH2CH2Cp), 2.20 (18H, SiCH2NH2+), 1.65 (72H, CqCH2CH2CH2Si), 1.17 (72H, CqCH2CH2CH2Si) 0.66 (72H, CqCH2CH2CH2Si), 0.22, 0.07 (216H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 174.18 (COO−), 155.70 (OCq arom of dendron), 140.79 (Cq arom), 127.67, 114.56 (CH arom), 73.10, 71.50, 71.19, 70.81, 69.31, 68.99, 68.28 (Cp-C), 65.99 (CH2NH2+CH2Cp), 50.38 (CH2NH2+CH2Cp), 43.42 (CqCH2CH2CH2Si), 42.32 (C q CH 2 CH 2 CH 2 Si), 30.00 (Si(CH 3 ) 2 CH 2 NH 2 + ), 18.26 (CqCH2CH2CH2Si), 17.76 (CqCH2CH2CH2Si), −1.73, −3.40 (Si(CH3)). Anal. Calcd for C639H867Si36Fe36N9O27: C, 63.28; H, 7.21. Found: C, 62.95; H, 7.12. Dendrimer 17. The ionic dendrimer 17 was synthesized from 7 (10 mg, 0.002 mmol, 1 equiv) and 13 (22.5 mg, 0.022 mmol, 9 equiv) using the general synthesis of ionic polyammonium carboxylate dendrimers. 1 H NMR (CDCl3, 300 MHz): δ (ppm) 7.16, 6.95, 6.82 (39H, arom), 4.42 (18H, CH2COO−), 4.34, 4.13, 4.06 (324H, Cp of Fc), 3.93 (18H, +NH2CH2Cp of Fc#), 3.28 (9H, C5(CH3)4H of Fc#), 2.23 (18H, SiCH2NH2+), 1.83−1.68 (216H, C5(CH3)4), 1.66 (72H, CqCH2CH2CH2Si), 1.22 (72H, CqCH2CH2CH2Si), 0.65 (72H, CqCH2CH2CH2Si), 0.21, 0.07 (216H, Si(CH3)2). 13C NMR (CDCl3, 75 MHz): δ (ppm) 174.18 (COO−), 155.70 (OCq arom of dendron), 140.79 (Cq arom), 127.55, 114.53 (CH arom), 86.31, 83.14, 81.88, 72.94, 71.19, 70.63, 68.11 (Cp-C), 67.85 (CH2COO−), 51.19 (CH2NH2+CH2Cp), 43.27 (CqCH2CH2CH2Si), 42.15 (C q CH 2 CH 2 CH 2 Si), 29.70 (Si(CH 3 ) 2 CH 2 NH 2 + ), 18.09 (CqCH2CH2CH2Si), 17.76 (CqCH2CH2CH2Si), −1.02, −3.20 (Si(CH3)2). Anal. Calcd for C711H1002Si36Fe36N9O27·2CH2Cl2: C, 64.39; H, 7.63. Found: C, 64.22; H, 7.85. 1

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S Supporting Information *

Text and figures giving general data, spectroscopic data for compounds, NMR and mass spectra, IR and DOSY NMR spectra, and cyclic voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail for D.A.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Helpful assistance and discussion with Christophe Deraedt (synthesis), Claire Mouche (mass spectrometry), and JeanMichel Lanier (NMR) from the CESAMO, Université Bordeaux 1, and financial support from the Université Bordeaux 1, the Centre National de la Recherche Scientifique (CNRS), the Agence Nationale de la Recherche (ANR), the Ministère de l’Enseignement Supérieur et de la Recherche Scientifique de Côte d’Ivoire (Ph.D. grant to R.D.), and the China Scholarship Council (Ph.D. grant to Y.W.) is gratefully acknowledged.

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REFERENCES

(1) (a) Newkome, G. R.; Moorefield, C. N.; Vögtle, F. Dendrons and Dendrimers. Concepts, Synthesis and Applications; Wiley-VCH: Weinheim, Germany, 2001. (b) Dendrimers and other Dendritic Polymers; Tomalia, D., Fréchet, J. M. J., Eds.; Wiley-VCH: New York, 2002. (c) Vögtle, F.; Richardt, G.; Werner, N. Dendrimer Chemistry. Concepts, Syntheses, Properties, Applications; Wiley-VCH: Weinheim, Germany, 2009. (d) Astruc, D.; Boisselier, E.; Ornelas, C. Chem. Rev. 2010, 110, 1857−1959. (e) Caminade, A.-M.; Turin, C.-O.; Laurent, R.; Ouali, A.; Delavaux-Nicot, B. Dendrimers. Towards Catalytic, Material and K


Organometallics

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